But even its size - based on observations from NASA's Spitzer Space Telescope - is uncertain. It may appear larger if it has a closely orbiting 'moonlet'.

It was first found by Australia's Siding Spring Observatory in 1981. It was named after Florence Nightingale, who invented modern nursing and developed new statistical techniques.

Astronomers have been using space radar and ground-based telescopes to observe the size and shape of the asteroid. This should be able to map its surface down to a resolution of 3m - outlining individual boulders and craters.

It is hoped figuring out its exact weight and shape could help refine calculations of its orbit.

In perspective ... Asteroid Florence, a 4.4km wide space rock, is flying past Earth at a distance some 18 times greater than the Moon.Supplied

While it orbits the Sun once every 2.35 years, it's not expected to come anywhere near this close to Earth again until 2500.

Only about 10 similarly-sized asteroids are known to cross Earth's orbit.

NASA has projected paths for all of them, and none are likely to hit - at least for several centuries to come.

If it did hit, it would be cataclysmic. Asteroids only 1km across are capable of destroying civilisation as we know it. One 4km across would be dramatically worse.